LICHEN - INTRODUCTION, COMPOSITION, MORPHOLOGICAL TYPES, THALLUS STRUCTURE, REPRODUCTION

            A. INTRODUCTION

            Lichens are dorsiventrally thallophytic plants which are formed due to close association of an alga and a fungus. The fungal component of lichen is called mycobiont and the algal component is called photobiont.  The two organisms live together in intimate connection forming a compound thallus called consortium.

            In a symbiotic association between the two partners, the fungus lives saprophytically on algal cells and obtains food from them. On the contrary alga gets benefited by fungi in providing water and nutrients. Fungus also protects algal cells from high light intensity.

            B. COMPOSITION OF LICHEN

            Lichen consists of two main components, photobiont and mycobiont.

            Photobiont:- The algal component of lichen is called photobiont. The photobiont is mainly Cyanophyceae and rarely Chlorophyceae. Altogether 37 genera have been identified as lichen photobiont (Hawksworth and Hill, 1984). These algae belong to blue-green (Cyanophyta) and green (Chlorophyta) algae. Lichen forming blue-green algae are Nostoc, Anabaena, Gloeocapsa, Stigonema, Chroococcus, Scytonema, etc. Common green algae are Chlorella, Cephaleuros, Trentepohlia, Trebouxia, etc.

            Mycobiont:- The fungal component of lichen is called mycobiont. The mycobiont mainly belong to Basidiomycotina and rarely Deuteromycotina. Most of the Ascomycetous lichens belong to Discomycetes or Pyrenomycetes. Dictyonema, Multiclavula, Omphalina, etc., are some of the species of lichen forming genera of Basidiomycotina. There are 55 species of lichen forming Deuteromycotina of which Blarneya hibernica is the most common.

            C. MORPHOLOGICAL TYPES OF LICHEN

            Lichens exhibit several morphological types. Hawksworth and Hill (1984), describe the following types of lichens –

            1. Leprose lichen:- This type is the simplest, here the thallus organization is very simple. The fungal hyphae doesnot envelop the algal cells throughout. This type of lichen grows superficially over the substratum and provides a powdery appearance and therefore is called leprose. Example – Leparia incana.

            2. Crustose lichen:- In this type, the lichen thallus is very closely adhered to the substratum and provides a crust-like appearance on rocks, soil, tree barks, etc. The algal cells are covered by a distinct layer of fungal cells. It is very difficult to separate the thallus from the substratum.

            Some variations of crustose lichens are as follows –

            (a) Placodioid – Here the entire surface of the thallus is radially striate and contains raised marginal tissues. Example – Lecanora, Caloplaca, etc.

            (b) Squamulose – Here the outer surface contains overlapping scale-like squamules. Example – Psora.

            3. Foliose lichen:- In this type, the thallus is flat, dorsiventral, leaf-like, well branched with irregular and lobed margin. The thallus is attached to the substratum with the help of rhizoid-like structure called rhizines. Externally they look like that of crinkled and twisted leaves. Example – Parmelia, Physcia, Peltigera, Collema, etc.

            4. Fruticose lichen:- In this type, the lichen thallus is vary in shape. They are well branched, erect or pedalous bushy which provide shrubby structure i.e., twig-like appearance. Example – Usnea, Cladonia, Letharia, etc.

            5. Filamentous lichen:- In this type, the algal component has the main role in the formation of the structure of the lichen thallus. The algal partner is well developed and filamentous which is covered by a few fungal hyphae only. The algal partner is dominant and as such named filamentous by Hawksworth and Hill (1984). Example – Ephebe, Racodium, Coenogonium, etc.

              D. STRUCTURE OF LICHEN THALLUS

            On the basis of internal structure of thallus, the lichens are divided into two groups namely homoiomerous and heteromerous lichens.

            1. Structure of homoiomerous lichen thallus:- The lichen thallus with the algal component scattered uniformly between the fungal hyphae throughout is called homioisomerous. In this type, the lichen thallus shows a simple structure with little differentiation. It consists of loosely interwoven mass of fungal hyphae with algal cells equally distributed throughout. Example – Collema, Leptogium, etc.

            2. Structure of heteromerous lichen thallus:- Most of the lichen belong to this category. They exhibit considerable differentiation and layered structure. A VS through a heteromerous lichen thallus shows the following structures –

            (a) Upper cortex:- It forms the upper surface which is generally thick and protective. The fungal hyphae in this region grow more or less vertically and are compactly interwoven to produce pseudoparenchymatous layer. The fungal hyphae are compactly arranged so that there is no intercellular space between the hyphae.

            (b) Algal zone:- It is the blue green or green zone which lies immediately beneath the upper cortex. It consists of a network of loosely interwoven fungal hyphae with the algal cells of green alga, intermixed with the fungal hyphae. The algal region is the photosynthetic region of the lichen thallus.

            (c) Medula:- It contains the central core of the thallus. It is less compact and consists of loosely interwoven hyphae with large spaces between them in certain regions. The fungal hyphae in this region are scattered and usually have thick walls. They run in all directions. The central hyphae of the medullary region usually run longitudinally.

            (d) Lower cortex:- It forms the lower surface of the thallus and is composed of densely compacted hyphae. They may run perpendicular to the surface of the thallus or parallel to it. Bundles of hyphae (rhizinae) often arise from the surface of the, lower cortex and penetrate the substratum to function as anchoring organs. In some lichen species the lower cortex is absent.

                 E. REPRODUCTION

            The common methods of reproduction in lichen are by vegetative, asexual and sexual methods –

            1. Vegetative reproduction:- Lichen thallus reproduce vegetatively by various methods. These are –

            (a) Fragmentation:- It consists in the breaking up of the thallus into segments which are distributed to start new growth. Fragmentation is brought about by ageing and accidental. In ageing the older cells in the basal part of the thallus die and separate the branches or lobes, each of which grows into new thallus. Sometimes larger portion of the thallus breaks accidentally from the parent thallus. Each of these broken parts develops into new individual.

            (b)  Soredia:- These are small, rounded granules or bud-lie outgrowths which develop in the form of a grayish-white or grayish-green powder in extensive patches usually over the upper surface or edges of the thalli of many species of lichen. Each soridium contains one or few algal cells closely surrounded by a little weft of fungal hyphae produced by branching of the hypha from the algal region.

            (c) Isidia:- These are small conical warts developed on the thallus of many lichens. Isidia are usually constricted at the base and thus can easily be broken off. Under the favourable conditions, each isidium grows into a new lichen thallus.

            Lichen thallus also reproduces vegetatively by phyllidia, blastidia, schizidia, hormocysts, goniocysts, etc.

            2. Asexual reproduction:- Lichen thallus reproduce asexually by various methods. These are –

            (a) Conidia:- In many lichens, conidia of different shape and size develop in multihypal structure known as conidiomata. The conidiomata are embedded in a flask-shaped structure, pycnidia. Each pycnidium appears to the exterior by means of ostiole. Conidiophores are developed from the inner lining of the wall of pycnidium. Conidia are produced terminally, laterally, or intercalary on conidiophores. Each conidium is colourlss and may be cylindrical or sickle-shaped or filiform in structure. Each conidium germinates by producing a germ hypha.

            (b) Oidia:- These are spores developed by breaking up of small fragments of lichen thallus. The oidium germinates to produce lichen thallus.

            3. Sexual reproduction:- Sexual reproduction takes place by the production of specialized male and female reproductive structures. Male reproductive structure is called spermogonium and the female reproductive structure is called carpogonium.

            (a) Spermogonia:- In certain species of lichens pycnidia-like structures are reported to function as spermogonia. Each spermogonium is a flask-shaped receptacle immersed in a small elevation on the upper surface of the thallus. It opens by a small pore, an ostiole, at the surface. The cavity of the spermogonium is filled with the sterile and fertile hyphae. The fertile hyphae produce minute rounded cells at their tips. These are the male cells and are called spermatia. They are non motile and are produced in large numbers. The spermatia are set free in a slimy mass which oozes out through ostiole.

            (b) Carpogonia:- The carpogonium is a cellular filament. It consists of two portions – the lower coiled portion and the upper straight portion. The coiled portion constitutes the ascogonium. It is multicellular and the cells are uninucleate. In certain species they are multinucleate. The ascogonium lies deep in the medullary region of the thallus. The carpogonia either develop from the hyphae or from the medullary region or from the hyphae deep in the algal layer of the lichen thallus. The straight upper portion of the carpogonium is called the trichogyne. It is also multicellular. The cells of trichogyne are elongated and septate. The terminal portion of the trichogyne ends in a long cell which projects beyond the surface of the thallus.

            (c) Fertilization:- At the time of fertilization, several spermatia are lodge to the sticky tip of the trichogyne. A few empty spermatia are found at the tip of the trichogyne  - this seems that their protoplast have migrated into the trichogyne. But the actual migration of the male nuclei down the trichogyne has not been seen.

            (d) Formation of ascocarp and ascospores:- After fertilization several ascogenous hypha develop from the basal portion of the ascogonium. Asci are produced at the ends of these freely branched ascogenous hyphae. Two nuclei in a young ascus fuse to form a diploid nucleus. This diploid nucleus divides and redivides to form eight haploid nuclei. The asci are uninucleate or binucleate. These asci bearing structures are ascocarps.

            Normally eight ascospores are formed in each ascus. They vary in colour, shape, size and structure. Ascospores are unicellular, transversely septate or both transversely and longitudinally septate.

            On liberation each ascospore germinates by producing hyphal branch. This hypal branch comes in contact with suitable alga, ultimately lichen thallus is formed after the combined growth of the fungus and the alga.

Prema Iswary,               

Assistant Professor,     

Department of Botany.

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